Ocular fundus auto imager

ABSTRACT

An ocular fundus imager ( 8 ) automatically aligns fundus illuminating rays to enter the pupil (P) and to prevent corneal reflections from obscuring the fundus image produced. Focusing the produced fundus image is automatically performed and is based upon the fundus image iself. A head restraint for the patient undergoing examination is in the form of a pair of spectacles which is not only easy to use accurately but significantly reduce the gross alignment between the optical system ( 8 ) and the patent&#39;s pupil (P).

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to the subject matter disclosedin a provisional application entitled “FUNDUS AUTO IMAGER”, filed Jul.17, 2000 and assigned Ser. No. 60/218,757 directed to an invention madeby the present inventor. This application is a continuation-in-part ofapplication Ser. No. 09/649,462, filed Aug. 25, 2000, now U.S. Pat. No.6,296,358, which claims priority from provisional Application No.60/218,757, filed Jul. 17, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of ocular imaging, and, moreparticularly, to devices for imaging the ocular fundus.

2. Description of Related Art

The term ocular fundus refers to the inside back surface of the eyecontaining the retina, blood vessels, nerve fibers, and otherstructures. The appearance of the fundus is affected by a wide varietyof pathologies, both ocular and systemic, such as glaucoma, maculardegeneration, diabetes, and many others. For these reasons, most routinephysical examinations and virtually all ophthalmic examinations includecareful examination of the ocular fundus.

Routine examination of the ocular fundus (hereinafter referred to asfundus) is performed using an ophthalmoscope, which is a small,hand-held device that shines light through the patient's pupil toilluminate the fundus. The light reflected from the patient's fundusenters the examiner's eye, properly focused, so that the examiner cansee the fundus structures.

If a hard copy of the fundus view is desired, a device called a funduscamera can be used. However, to use existing fundus cameras successfullyis a very difficult undertaking. The operator must (1) position thefundus camera at the correct distance from the eye, (2) position itprecisely in the vertical and horizontal directions in such a way thatthe light properly enters the pupil of the patient's eye, (3) refine thehorizontal and vertical adjustments so that the light reflected from thefront surface of the eye, the cornea, does not enter the camera, (4)position a visual target for the patient to look at so that the desiredregion of the fundus will be imaged, and (5) focus the fundus image. Allthese operations must be performed on an eye that is often moving.Therefore, the use of existing fundus cameras requires a significantamount of training and skill; even the most skilled operators oftencollect a large number of images of a single eye in order to select onethat is of good quality.

In existing fundus cameras, alignment and focusing are performed undervisual control by the operator. This usually requires that the patient'seye be brightly illuminated. Such illumination would normally cause thepupils to constrict to a size too small to obtain good images.Therefore, most existing fundus cameras require that the patient's pupilbe dilated by drugs.

U.S. Pat. No. 4,715,703 describes an invention made by one of thepresent inventors and discloses apparatus for analyzing the ocularfundus. The disclosure in this patent is incorporated herein byreference.

SUMMARY OF THE INVENTION

The present invention is in the nature of a fundus camera whichautomatically and quickly performs all the aligning and focusingfunctions. As a result, any unskilled person can learn to obtain highquality images after only a few minutes of training and the entireimaging procedure requires far less time than existing fundus cameras.Moreover, all of the automatic aligning and focusing procedures areperformed using barely visible infrared illumination. With suchillumination, the patient's pupils do not constrict and for all butpatients with unusually small natural pupils, no artificial dilation isrequired. The fundus images can be obtained under infrared illuminationand are acceptable for many purposes so that the patient need not besubjected to the extremely bright flashes required for existing funduscameras. To obtain standard color images using the present invention, itis sometimes necessary to illuminate the eye with flashes of visiblelight. However, such images can be obtained in a time appreciablyshorter than the reaction time of the pupil, so that the pupilconstriction that results from the visible flash does not interfere withimage collection. Unlike existing fundus cameras, the present inventionprovides for automatic selection of arbitrary wavelengths of theilluminating light. This facility has two significant advantages. First,it is possible to select illuminating wavelengths that enhance thevisibility of certain fundus features. For example, certainnear-infrared wavelengths render the early stages of maculardegeneration more visible than under white illumination. Second, bycareful selection of two or more wavelengths in the near infrared, it ispossible to obtain a set of images which, when properly processed,generate a full color fundus image that appears very similar to a colorimage obtained with white light. Thus, it is possible to obtainacceptable color fundus images without subjecting the patient to brightflashes.

It is therefore a primary object of the present invention to provide afundus imager which automatically positions fundus illuminatingradiation to enter the pupil while preventing reflection from the corneafrom obscuring the fundus image, irrespective of movement of the eye orthe patient's head within the head restraint.

Another object of the present invention is to provide automatic focusingof the fundus image based upon the image itself.

Yet another object of the present invention is to provide automaticpositioning of one or a sequence of fixation targets to select thesections(s) of the fundus to be imaged.

Still another object of the present invention is to provide a fundusimager for collecting a set of images that can be arranged in a montageto provide a very wide angle fundus image facilitated by the capabilityof the fundus imager to automatically align and focus the images.

A further object of the present invention is to provide automaticsetting of video levels in a fundus imager to use the full range oflevels available.

Yet another object of the present invention is to permit aligning andfocusing a fundus imager under infrared illumination to permit imagingwithout drug induced dilation of the pupil.

A yet further object of the present invention is to provide forautomatic selection of illumination wavelength.

A yet further object of the present invention is to provide a coloredimage from a fundus imager by sequential imaging and registration ofimages.

A yet further object of the present invention is to provide anapparently normally colored image generated by two infrared wavelengths.

A yet further object of the present invention is to provide forautomatic acquisition by a fundus imager of a stereo image pair having aknown stereo base.

A yet further object of the present invention is to provide a headpositioning spectacle frame for use with a fundus imager.

A yet further object of the present invention is to accommodate forastigmatism and/or extreme near and far sightedness by placing a lens ofthe patient's glasses in the path of illumination of the fundus imager.

A yet further object of the present invention is to provide a method forautomatically positioning the illuminating radiation of a fundus imagerto prevent corneal reflections from obscuring the fundus image obtained.

A yet further object of the present invention is to provide a method forautomatic focusing in a fundus imager.

These and other objects of the present invention will become apparent tothose skilled in the art as the description thereof proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with greater specificity andclarity with reference to the following drawings, in which:

FIG. 1 is a schematic diagram illustrating the functional elements ofthe present invention;

FIG. 2 illustrates the location of the photo detectors relative to lensL3;

FIG. 3 is a block diagram illustrating a representative computer systemfor operating the present invention;

FIG. 4 illustrates the effect of corneal reflections to be avoided;

FIG. 5 is a schematic illustrating an alignment of the optical axis toavoid corneal reflections;

FIG. 6 is a graph illustrating determination of an acceptable videolevel;

FIG. 7 illustrates determination of edge points;

FIGS. 8A, 8B and 8C depict the light rays from a point to an image planewithout an interposed aperture, and with an interposed aperture at twolocations displaced from one another;

FIGS. 9A and 9B illustrate the shift of an image upon an image planelocated beyond the focal plane in response to displacement of aninterposed aperture from one location to another;

FIGS. 10A and 10B illustrate the shift of an image upon an image planelocated short of the focal plane in response to displacement of aninterposed aperture from one location to another;

FIG. 11 illustrates a head restraint in the form of a pair ofspectacles;

FIG. 12 illustrates a first variant for realigning the optical axis;

FIG. 13 illustrates a second variant for realigning the optical axis;and

FIG. 14 illustrates a third variant for realigning the optical axis.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated a preferred embodiment ofoptical system 8 of the present invention. Lens L1 focuses light from alight source S onto a small aperture A1. The light source may be asource of visible light, infrared radiation or of a wavelength in thenear visible infrared region. Light passing through aperture A1 passesthrough a filter F and is reflected by mirror M1 toward lens L2. MirrorM1 is pivotally mounted to permit rotation about two orthogonal axes,which pivotal mounting is represented by device 10 attached to themirror. Lens L2 collimates (makes parallel) light from aperture A1. Abeam splitter BS1 transmits about ninety percent (90%) of the incidentlight from lens L2 to lens L3. Half of the light passing through lens L3is reflected by beam splitter BS2 and is absorbed by light trap LT. Theother half of the light passing through lens L3 forms an image ofaperture A1 in the focal plane of lens L3, which focal plane lies in theplane of a patient's pupil P. The light passing through the pupililluminates a section 12 of ocular fundus 14 (hereinafter only the termfundus will be used).

Light diffusely reflected from fundus 14 emerges from pupil P and halfof it is reflected by beam splitter BS2 toward collimating lens L4,which lens is at its focal distance from the pupil. An infrared lightemitting diode (LED), representatively shown and identified by referencenumeral 16, diffusely illuminates the region of the front of the eye.About ten percent (10%) of the light is transmitted through beamsplitter BS3, which light passes through lens L5. Lens L5 forms an imageof the pupil and the front of the eye in the plane of a video sensor C1.The video output from video sensor C1 is displayed on an operator'smonitor (on computer screen shown in FIG. 3) to provide a view of theeye and of the pupil.

If the patient's eye is focused at infinity, the light reflected fromeach point on fundus 14 will be collimated as it is incident on lens L4.Therefore, 90% of the light reflected from beam splitter BS3 will forman aerial image of the fundus in the focal plane of lens L4, which focalplane is represented by a dashed line identified as FI (Fundus Image).The light passes through lens L6, which lens is at its focal distancefrom fundus image FI. Thus, lens L6 will collimate light from each pointon the fundus. Further, because the light considered as originating inthe plane of pupil P is collimated by lens L4, lens L6 will form animage of the pupil in its back focal plane, which is coincident with thelocation of second aperture A2. Light passing through second aperture A2is incident on lens L7, which lens will then form an image of the fundusin its back focal plane which is coincident with second video sensor C2.The video image produced by video sensor C2 represents an image of thefundus.

If the eye is not focused at infinity, the aerial fundus image FI willbe moved away from the back focal plane of lens L4. For example, if theeye is nearsighted, the aerial fundus image will move toward lens L4.Such movement would cause the fundus image to be defocused on videosensor C2. Focusing the image under these conditions is accomplished asfollows. Lens L6, aperture A2, lens L7, and video sensor C2 aremechanically connected to one another by a focusing assembly labeled FA;that is, these elements are fixedly positioned relative to one anotherand move as a unit upon movement of the focusing assembly. A unitidentified by reference numeral 18 provides rectilinear movement of thefocusing assembly on demand.

A set of photodetectors PD, of which three are shown in FIG. 1, lie in aplane between the eye and beam splitter BS2. As further shown in FIG. 2from the viewpoint of the eye, orthogonal pairs of photodetectors arelocated in the vertical and horizontal axes relative to lens L3. Thepurpose of these photodetectors is that of sensing any light diffuselyreflected from the iris.

The entire optical system (8) discussed above and illustrated in FIG. 1is supported upon an assembly identified by reference numeral 20. Theassembly includes motive elements, such as rectilinear actuators andrelated servomechanisms responsive to commands for translating theentire optical system horizontally (laterally), vertically and towardand away from the eye, as representatively depicted by set of arrows 22.

To operate optical system 8, a computer control system 30 is required,which is representatively illustrated in FIG. 3. The computer controlsystem includes a central processing unit (CPU) 32, such as amicroprocessor, and a number of units interconnected via a system bus34. A random access memory (RAM) 36, a read only memory (ROM) 38 areincorporated. An input/output adapter 40 interconnects peripheraldevices, such as a disk storage unit 42. A user interface adapter 44connects the keyboard 46, a mouse (or trackball) 48, a speaker 50, amicrophone 52, and/or other user interface devices, such as a touchscreen (not shown) with system bus 34. A communication adapter 54interconnects the above described optical system 8 through acommunication network 56. A display adapter 58 interconnects a displayunit 60, which maybe a video screen, monitor, or the like. The computeroperating system employed maybe any one of presently commerciallyavailable operating systems.

In operation, an operator enters patient information data into thecomputer control system using the keyboard and also enters the locationor set of locations on the fundus that is/are to be imaged. It may benoted that the field of view of the optical system is preferably 30° indiameter while the ocular fundus is about 200° in diameter. To imagevarious regions of the 200° fundus, the eye can be rotated with respectto the optical system; such rotation is achieved by having the patientlook from one reference point to another. After entry of the raw data,the patient's head is juxtaposed with a head positioning apparatus tolocate the eye in approximate alignment with respect to the opticalaxis. An image of the front of the eye produced by video sensor C1appears on computer screen 60. The operator may use a trackball or mouse48 or similar control to move the image horizontally and verticallyuntil the pupil is approximately centered on a set of cross-hairsdisplayed on the computer screen. A further control is used to focus theimage of a pupil. Such horizontal and vertical movements, along withfocusing of the image of the pupil, are achieved by moving the entireoptical system 8 through energization of assembly 20 (see FIG. 1). Thatis, the horizontal and vertical movements of the image are achieved bymoving the entire optical system horizontally and vertically and thefocusing of the pupil image is accomplished by moving the entire opticalsystem toward or away from the eye. When the operator is satisfied thatthe pupil image is in sharp focus and that the pupil is approximatelycentered, the operator de-energizes LED 16 (which illuminated the frontof the eye) and then initiates the automatic alignment and imagecollection procedure.

To achieve proper alignment of the optical system with the eye requiresthat the light from light source S enter the pupil. Initially, theangular position of mirror M1 is set so that the image of aperture A1lies on the optical axis of the system. It is noted that the image ofaperture A1 contains the light used to illuminate the fundus. Sincevideo sensor C1 also lies on the optical axis, if the operator hasinitially centered the pupil image even crudely, light from light sourceS will enter the pupil. About three percent (3%) of the light incidenton the eye will be reflected from the corneal surface and if this lightreaches video sensor C2, it would seriously obscure the image of thefundus. Therefore, the optical system includes the following elementsfor preventing corneal reflection from reaching video sensor C2.

If the light rays forming the image of aperture A1 were aligned so thatthe central ray were perpendicular to the corneal surface, then many ofthe rays in the corneal reflection would pass backward along theincident light paths. As shown in FIG. 4, the central ray would passback on itself; the ray labeled Ray-1 would pass back along the path ofthe incident ray labeled Ray-2, etc. (The angle at which a ray isreflected from a shiny surface can be determined as follows. First, findthe line that is perpendicular to the surface at the point that the rayhits. Then find the angle between the incident ray and the perpendicularray; this is called the “angle of incidence”. Finally, the ray will bereflected at an angle equal to the angle of incidence but on the otherside of the perpendicular line. This is called the angle of reflection.)It is therefore evident from the schematic shown in FIG. 4 that manyrays reflected from the corneal surface and impinging upon beam splitterBS2 would enter lens L4 and impinge upon video sensor C2.

However, the corneal surface is steeply curved and if the central ray ofthe incident light is moved far enough away from the perpendicular tothe cornea, as shown in FIG. 5, the reflected light will be deflectedfar enough to miss beam splitter BS2 and therefore miss passing throughlens L4 and therefore not impinge upon video sensor C2. The method forachieving this deflection will be described below.

The image of aperture A1 is appreciably smaller than the smallest pupilfor which optical system 8 will operate correctly. In the preferredenvironment, the smallest useful pupil is four millimeters (4 mm) indiameter and the image aperture A1 is one millimeter (1 mm) in diameter.Initially, the image of aperture of A1 lies on the optical axis and isthus approximately centered on the pupil. Mirror M1 is actuated bysignals generated by the computer system to rotate about a vertical axisto cause the image of aperture A1, and thus the light that illuminatesthe fundus, to move horizontally, laterally in small increments (e.g.0.1 millimeters), to the left across the pupil. When the image ofaperture A1 just begins to fall beyond the pupil, that is to fall uponthe iris, the light scattered by the iris will fall on all four photodetectors PD (see FIG. 2). The photodetectors become enabled to generatea signal supplied to the computer system indicative of such event.Thereafter, the image of aperture of A1 is moved backward one step and avideo image of the fundus is saved. Mirror M1 is then moved back to thecenter and stepped in increments toward the top of the pupil until thephoto detectors indicate light reflected from the iris. Thereafter, theimage of aperture A1 is moved backward one step and a video image of thefundus is saved. This step is repeated for each of the right and bottomedges of the pupil.

The computer now contains four fundus images taken with light at fourlocations at the edge of the pupil. If the corneal reflection hasreached video sensor C2 in one of those images, the amount of lightforming that image will be greater then the light forming the otherimages. The computer system examines each of the four images and selectsthe one for which the average video level is lowest. This image ispresumed not to contain light from any corneal reflection. It may benoted that the geometry of the cornea and pupil are such that for apupil four millimeters (4 mm) or larger, the corneal (but not iris)reflection will always be absent from at least one of the four images.

When the image of aperture A1 falls on the iris, the diffuse reflectionwill illuminate all four detectors. Most of the time that the image ofaperture A1 falls on the pupil and not the iris, the corneal reflectionwill illuminate one or two photo detectors. However, the cornealreflection will never fall on all four detectors. Therefore, to achievethe goal of placing the image of aperture A1 into the pupil, it isnecessary to determine location of the edge of the pupil by moving theimage of aperture A1 until all four of the four detectors simultaneouslygenerate a signal indictive that they are illuminated.

The four edges of the pupil are located as a function of the signalsgenerated by the photodetectors, as described above. From the locationof these edges, the center of the pupil can be determined by thecomputer system with respect to the optical axis of the instrument. Ifthe center of the pupil does not lie approximately on the optical axis,the computer system commands the horizontal and vertical motors(assembly 20) to move the entire optical system 8 until the pupil iscentered. The servomechanisms actuating the horizontal and verticalmotors are slow compared to the motions of mirror M1. Theseservomechanisms are intended to permit limiting the motions of mirror M1within a restricted range to reduce the sizes of the entrance and exitpupils of the optical system and to simplify the optical design of thelenses. In this way, light is continuously and automatically introducedthrough the pupil to illuminate the fundus and images contaminated bylight reflected from the cornea surface can be automatically discarded.

An alternative method for tracking the pupil and positioning the imageof aperture A1 on the pupil of the eye will be described hereafter. Inthe above described procedure, an image of the patient's pupil is formedon video sensor C1. The image was used by the operator to perform roughalignment of the optical system with the eye. However, image appearingon video sensor C1 can also be used for automatic tracking of the eyeand the positioning of the image of aperture A1. This is done by usingthe computer system for extracting the edges of the pupil from the videosignal and computing the coordinates of its center and of its edges.

When the image of aperture A1 falls within the pupil, the light itcontains passes through the pupil, falls on the fundus and is scatteredby the fundus. Some of that scattered light exits the pupil. Thus, whenthe image of aperture A1 falls within the pupil, the pupil isbacklighted by light reflected from the fundus, and an image of thepupil on video sensor C1 consists of a bright disk on a dark background.The goal is to determine the location of the center and of the edges ofthis image so that aperture A1 can be automatically placed where thefundus will be illuminated and the image of the fundus on video sensorC2 will then not be spoiled by light reflected from the cornea. If thepupil is correctly centered on the optical axis of the optical system,the pupil image will be centered. If not, the direction and distancebetween the center of the pupil and the center of the field of view ofthe camera can be used to drive servomechanisms (assembly 20 in FIG. 1)to correct the error by moving the entire optical system. Further, ifthe edges of the pupil are located, those locations can be used toposition the image of aperture A1 just inside the edge of the pupil.There are a number of ways of finding the center and the edges of thepupil image from the video signal produced.

A method for finding the center and the edges of the pupil image willnow be described. It involves finding the edges of the pupil image oneach video line that intersects the edges and then computing the mostlikely position of the center and of the edges of the actual pupil. Theimage from video sensor C1 is read out, as is the standard videopractice, by reading the values of the various points along a horizontalline and then the values along the next horizontal line, etc.(neglecting the detail of interlacing). If a given video horizontal lineintercepts the image of the pupil, the video level will abruptly risefrom the dark background level to the brighter level of the pupil. Tolocate this transition and find the position of each edge, it isnecessary to define the values of the background and of the pupil. To dothis, a histogram of pixel values is formed during the first few videoframes. It will contain a large peak with values near zero, representingdark background pixels, and additional peaks at higher values thatrepresent the pupil and various reflections to be discussed below. Atypical histogram is illustrated in FIG. 6. Each point along thehorizontal axis represents a different video signal level and each pointon the vertical axis indicates the area of the image that displays thecorresponding video level.

The “background level” is defined as the level just below the firstminimum. Specifically, the histogram is first smoothed using a runningblock filter. That is, for a position on the horizontal axis thevertical value on the curve is replaced by the average of the verticalvalue and its adjoining values. This computation is performed in stepsalong the horizontal axis (video level) until there are ten consecutivevalues for which the vertical axis increases. The “background value” isthen defined as the lowest of these ten values. An “edge point” on eachhorizontal line is defined as the horizontal location for which thevideo level changes from equal to or below the “background value” toabove that value or changes from above that value to equal or below thatvalue. As the video scan proceeds, the location of each point is saved.Thus, at the end of each video frame, a set of point locations is storedin the computer memory (see FIG. 3).

If the pupil image consists solely of a bright disk on a darkbackground, the above described procedure would essentially always besuccessful in finding a close approximation to the actual pupil edges.However, for real pupil images the procedure is confounded by twosources of reflections. First, light reflected from the cornea; if thislight reaches video sensor C1, it will form a bright spot superimposedon the pupil image. If that spot were entirely within the margins of thepupil, it would not interfere with the process described above. However,if it falls on the edge of the pupil image, as it may when a patient islooking at an angle to the optical axis of the optical system, then itwill appear as a bulge on the edge of the pupil, as illustrated in FIG.7. Therefore, some of the “edge points” located by the abovecomputations will actually be edges of the corneal reflection instead ofthe edge of the pupil. Second, a similar problem arises if the image ofaperture A1 falls on the edge of the pupil, as it might during an eyemovement too fast to be accurately tracked and compensated. In thatevent, finding the center and the edges of the pupil requires specialprocedures.

One such special procedure will described below. The edge points arecollected as described above. There will typically be several hundredsuch points. An ellipse is then found (determined) that best fits theset of edge points. The pupil of the human eye is usually circular, butif it is viewed from an angle, as it will be if the patient is lookingat a point other than on the optical axis, then the image of the pupilwill approximate an ellipse. So long as the reflections from the corneaand iris do not overlap a major part of the pupil edge (and so long asthe pupil is not of grossly abnormal shape), such a procedure yields agood estimate of the locations of the actual pupil center and the edge.

One method for finding the best fitting ellipse will be described.Assuming that 200 hundred points have been labeled edge points by theabove procedure, each of such points has a horizontal (x) and a vertical(y) location. Assume that these 200 hundred points, that is pairs ofvalues (x,y), are in a consecutive list. Five points are selected atrandom from the list, requiring only that each selected point beseparated from the next selected point by ten or so positions on thelist. This process will then yield the locations of five putative edgepoints that are some distances apart on the pupil. These five pairs ofvalues are substituted into the equation for an ellipse and solved forthe five ellipse parameters. One form of equation for an ellipse is:c 1*x^2+c 2 *xy+c 3 *y^2+c 4 *x+c 5 *y=1Substitute the five putative edge points as the pairs (x,y) of values inthat equation. Invert the matrix to find the values for c1 through c5.Then the angle that the ellipse makes with the xy axis is:θ=½*arc cot((c 1 −c 3)/c 2)Then if u=x*cos θ+y*sin θ and v=−x*sin θ+y*cos θ, thend1*u^2+d3*v^2+d4*u+d5*v=1Where d1=c1*cos ^2θ+c2*cos θ*sin θ+c3*sin ^2θ

d3=c1*sin ^2θ−c2*cos θ*sin θ+c3*cos ^2θ

d4=c4*cos θ+c5*sin θ

d5=−c4*sin θ+c5*cos θ

The center of the ellipse has u coordinate u=−d4/(s*d1) and v coordinateV=−d5/(2*d3) so the center of the ellipse has the x coordinatex=u*cos θ−v*sin θand the y coordinatey=u*sin θ+v*cos θIf R=1+d4^2/2d1+d5^2/2d3 then the semiaxes of the ellipse have lengths

Square root (R/d1) and square root (R/d3)

This entire procedure is repeated, say, 100 times for 100 different setsof putative points yielding 100 different estimates of the x,y locationof the center. The best fitting ellipse is the one for which the centeris closest to the median x and y values of the set of 100.

The resulting deviations between the horizontal and the verticallocations of the center of the chosen ellipse and the optical axis ofthe optical system can be used directly as error signals to drive thepositioning servos associated with assembly 20 and the image of apertureA1 can be directly and finely positioned such as by moving mirror M1 sothat the image lies just inside the pupil.

The corneal reflections can be prevented from spoiling the image of thefundus by the following procedure. The method involves directing thepatient's line of sight to certain selected positions. If the selectedposition is straight ahead, that is, the line of sight is directed alongthe optical axis, then positioning the image of aperture A1 in anydirection at the margin of the pupil will cause the corneal reflectionto be sufficiently deflected (assuming a pupil of 4 mm diameter orlarger). If the selected position is in any other direction, thenpositioning the image of aperture A1 on the same side of the pupil willcause the reflections to be sufficiently deflected (see FIG. 5). Forexample if the patient is looking to the left and the image of apertureof A1 is positioned at the left margin of the pupil, the cornealreflection will be deflected far enough to miss lens L4.

An automatic focusing method will be described with reference to FIG. 1.Aperture A2 is a hole significantly smaller then the image of the pupiland which is conjugate with the pupil; that is, it is in the same planeas the image of the pupil. In the preferred embodiment, aperture A2 is arectangular aperture one millimeter (1 mm) wide and two millimeter (2mm) high. Aperture A2 is mounted on a linear actuator 24 that can moveit rapidly in a horizontal direction, as depicted by arrows 26. In thealignment method described above, the image of aperture A1 is made tolie near the edge of the pupil and a fundus image is saved. To focus,two images are saved in rapid succession, one with aperture A2 lying tothe right of the center of the pupil image and the second with apertureA2 lying to the left of the center of the pupil image, by enablinglinear actuator 24. If the focusing assembly FA is positioned so thatthe fundus image FI lies in the focal plane of lens L6 (the fundus imageis thus correctly focused on video sensor C2) then the two images takenwith aperture A2 in each of its two positions will be in registry andsuperimposable. However, if focusing assembly FA is not correctlypositioned and the image is out of focus, then one of the images will behorizontally displaced with respect to the other. With the particularoptical arrangement illustrated in FIG. 1, the direction of thedisplacement indicates the direction that focusing assembly FA must moveto achieve correct focus and the size of that displacement is directlyproportional to the distance the focusing assembly must move to correctfocus.

To explain more clearly the direction of displacement of the focusingassembly (FA) to achieve correct focus, joint reference will be made toFIGS. 8A, 8B, 8C, 9A, 9B, 10A and 10B. As shown in FIG. 8A, lens Lxforms an image of a point P that is sharply focused on image plane IP.If the aperture of an apertured plate Ax is placed between point P andlens Lx off the optical axis, the image of point P will be in focus onimage plane IP, as shown in FIG. 8B. However, because certain of therays are excluded by the plate, the intensity of the image on the imageplane will be reduced. As depicted in FIG. 8C, displacement of theaperture in apertured plate Ax will have no effect upon the location ofthe image of point P on the image plane. If the image plane IP isdisplaced from the focal plane FP, as depicted in FIG. 9A, a blurredimage of point P will appear on the image plane at a locationdiametrically opposed relative to the optical axis from the aperture inapertured plate Ax. When the apertured plate is displaced (like thedisplacement shown in FIG. 8C), the blurred image on the image planewill be displaced in a direction opposite from the displacement of theapertured plate, as shown in FIG. 9B. If image plane IP is short of thefocal plane FP, as shown in FIG. 10A, the rays passing through theaperture of apertured plate Ax will form a blurred image of point P onthe image plane. This blurred image will be on the same side of theoptical axis as is the aperture. If the apertured plate is displaced(like the displacement shown in FIG. 8C), the blurred image of point Pon the image plane will be displaced in the same direction, as shown inFIG. 10B. From this analysis, the following conclusions are evident. Ifthe image is in focus on the image plane, any shift of an aperturedplate will not affect the position of the image in the image plane. Ifthe image plane is beyond the focal plane, the image on the image planewill shift in a direction opposite to the direction of displacement ofthe aperture. Congruously, if the focal plane is beyond the image plane,the image on the image plane will shift in the same direction as theaperture is displaced. From these relationships, it is a simplecomputational exercise performable by the computer system illustrated inFIG. 3 to determine the direction and amount of displacement of focalassembly FA necessary to place the image of the fundus in focus on videoscreen C2.

Thereby, automatic focusing is achieved by finding the displacement ofone image of a pair of images that is required to bring the two imagesinto registry and then moving the focusing assembly in accordance withsuch result. The required displacement can be found by computing across-correlation function between the two images. This is amathematical computation that, in effect, lays one image on top of theother, measures how well the two images correspond, then shifts oneimage horizontally a little with respect to the other, measures thecorrespondence again, shifts the one image a little more and measuresthe correspondence again and repeats these steps for a large number ofrelative positions of two images. Finally, the shift that produces thebest correspondence is computed.

Even when a patient is trying to hold his/her eye steady, the eye isalways moving and as a result the fundus image is continually shiftingacross the sensing surface of video sensor C2. Exposure durations forindividual images are chosen to be short enough (about 15 milliseconds)that this motion does not cause significant blur. Nevertheless, the timeinterval between members of pairs of images taken during the automaticfocusing procedure may be long enough to allow movement between theimages that would confound the focusing algorithm. Therefore, the actualprocedure requires that a number of pairs of images be collected and theaverage displacement computed as the measure of focus error.

Selection of the fundus region to be imaged will now be described.Adjacent beam splitter BS 1 illustrated in FIG. 1 lies a set of dotslabeled FIX. Each dot represents a visible light emitting diode (LED).Beam splitter BS1 reflects about 10% of the light from these LED'stoward lens L3 and the eye. The set of dots (FIX) lies in the back focalplane of lens L3 and these LED's appear to the eye as if they were along distance away. Only one of the LED's is illuminated at any giventime and the patient is asked to look at it. When the patient looks atthe illuminated LED, the location of the LED with respect to the opticalaxis of the instrument determines the location on the fundus that willbe illuminated and imaged. For example, if the LED that lies on theoptical axis is turned on and the patient fixates it, then the imagewill be centered on the fovea or macula. If the illuminated LED is 17°to the patient's left, then the region of the fundus imaged has itscenter 17° to the left of the macula (as observed from the front of theeye).

In addition to the LED's in the plane labeled FIX, other visible LED's,such as LED 28 shown in FIG. 1, are positioned at various angulardisplacements from the optical axis, lying, such as to the sides of lensL3. When one of these LED's is turned on, it does not appear at opticalinfinity but nevertheless the patient can successfully fixate it toyield a view of more peripheral fundus features.

When the operator sets up the instrument prior to collecting images,he/she selects the region or set of regions of the fundus to be imaged.If just one region is to be imaged, the appropriate LED will be lighted.If a series of locations is to be imaged, the computer (see FIG. 3)automatically selects the LED corresponding to the first location; afterthe image has been collected, the remaining selected LED's are lightedin sequence until the desired sequence of images has been obtained. Ifsuch a sequence involves locations that are widely separated so that thepatient must make a significant eye movement to refixate, then thecomputer commands the horizontal and vertical positioning servomechanisms of assembly 20 to move the optical system 8 (and opticalaxis) to the position where the center of the pupil is expected to beafter the fixation movement. This eases the task of the servo loop thatacts on mirror M1 and its limited repositioning capability.

After the image of aperture A1 has been located to exclude the cornealreflection and focusing has been achieved, another pair of images iscollected with aperture A2 in each of two positions. This pair of imagesconstitutes a stereo pair of images with a known stereo base, which baseis the distance through which aperture A2 has moved.

During the alignment and focusing procedures previously described,filter F (see FIG. 1) blocks visible light but transmits near infraredwavelength radiation. To obtain an image or set of images in infraredillumination, this filter need not be changed. For certain forms ofcolored images, it is necessary to collect an image, rapidly change thefilter to one transmitting a different wavelength band, acquire anotherimage and return the infrared filter. The result is two or more images,each taken in a different wavelength band. To display a single colorimage, the different images are used to drive different color guns in adisplay device. For example, if one image is collected in redillumination and a second is collected in green illumination then thered image is made to drive the red gun in the display device and thegreen image is made to drive the green gun in the display device. Thecombined images will appear as a normal (two color) image.

During the interval between images collected in different wavelengths,it is possible that the eye, and thus the fundus image, will movesignificantly. If such movement occurs, then the variously coloredimages would not be in registry when displayed. To prevent thisoccurrence the images are automatically registered before beingdisplayed by performing a two-dimensional cross-correlation and thenshifting the images in accordance with the result.

Essentially all standard ophthalmic instruments position a patient'shead using a combination of a chin rest and a forehead rest. Typically,the chin rest is manually raised or lowered to bring the eye to aboutthe right height. The present invention employs a different apparatusfor positioning a patient's head and therefore eye. As shown in FIG. 11,a frame 70 of a standard pair of spectacles is mounted on the case orother support of the instrument (not shown); preferably, temple pieces72, 74 are omitted and therefore shown in dashed lines. The patient isinstructed to place his/her head in the frame of the spectacles. Becausespectacles are so familiar, little instruction is actually needed. Thevariations in the location of the eyes with respect to the bridge of thenose is such that virtually all eyes will fall within a cube that isfixed with respect to the instrument and is about 20 millimeters on aside. This is a much smaller variation then is encountered by using theusual chin and forehead rest apparatus. Thus, the commonality anduniformity of the location of the eyes with respect to the spectaclesrequires a very small range of accommodating movement of optical system8. Furthermore, properly chosen spectacle frames constrain head movementappreciably better than a chin and forehead rest apparatus; thus, therequirement for an automatic tracking system is reduced.

The motion of focusing assembly FA (see FIG. 1) compensates for apatient's spherical refractive error (near or farsightedness) but doesnot correct for astigmatism. Because the fundus images are collectedthrough a small aperture A2, moderate amounts of astigmatism will notsignificantly spoil the image quality. If a patient has a strongastigmatism, correction is desirable. In principle, this correctioncould be achieved by allowing the patient to wear his/her glasses in theinstrument. However, the reflections from such eyeglasses wouldseriously impair the image quality. An equivalent result which does notcreate serious reflections is that of mounting the patient's eyeglassesin the optical system in a plane close to the plane of aperture A2 withthe same orientation as when worn. A representative mounting 29 forreceiving and retaining a lens of a pair of glasses is shown in FIG. 1.

In the embodiment of the optical system 8 shown in FIG. 1, the image ofaperture A1 is made to move by two axis movement of mirror M1. Analternative apparatus is shown in FIG. 12. One end 80 of an opticalfiber or fiber bundle 82 is illuminated by light source S through lensL1. The other end 84 of the fiber bundle serves the same function asaperture A1. Linear actuators, representatively depicted by blocks 86,88, engage end 84 via elements 90, 92, respectively, to enabled rapidmovement of end 84 in the vertical and horizontal directions to realignlight path 94 emanating therefrom and passing through lens L2 to beamsplitter BS 1. Alternatively, if the amount of light from source S maybe small, an infrared LED may provide sufficient illumination for alloperations, including imaging. In such event, source S and fiber bundle82 shown in FIG. 13 may be eliminated by mounting the infrared LED inthe position occupied by end 84 of the fiber bundle. This configurationis illustrated in FIG. 12 and includes actuators 86, 88 with theirrespective elements 90, 92 for repositioning the LED vertically andhorizontally, as may be required to align the optical axis.

Another method/apparatus for providing a small illuminating spot thatcan be moved with respect to the pupil is to substitute for the LEDshown in FIG. 14 a set of four LED's 100, 102, 104 and 106, as shown inFIG. 13. LED 100 is attached to an actuator 110 via an element 112 toreposition the LED vertically, as represented by arrow 114. Actuator 116is connected to LED 102 via element 118 to bring about horizontalmovement, as depicted by arrow 120. Actuator 122 is connected to LED 104via element 124 to bring about vertical movement of the LED, as depictedby arrow 126. Actuator 128 is connected to LED 106 via actuator 130 tobring about horizontal movement, as depicted by arrow 132. In operation,a selected one of LED's 100, 102, 104 and 106 is illuminated andrepositioned so that its image along the optical path falls just insidethe edge of the pupil.

While the invention has been described with reference to severalparticular embodiments thereof, those skilled in the art will be able tomake the various modifications to the described embodiments of theinvention without departing from the true spirit and scope of theinvention. It is intended that all combinations of elements and stepswhich preform substantially the same function in substantially the sameway to achieve the same result are within the scope of the invention.

1. Apparatus for imaging the ocular fundus, said apparatus comprising incombination: a) a source of illumination for illuminating at least asection of the ocular fundus of a patient; b) means for directing theillumination along an optical axis to enter the pupil of the patient; c)a video sensor responsive to an image of the pupil reflected from theeye of the patient for depicting the alignment of the optical axis withthe pupil of the patient; d) a beam splitter and a lens for directingthe reflected image of the ocular fundus toward said video sensor andfor creating a reflected image, respectively; e) positioning means foraligning the optical axis in response to the depicted alignment; f) afocusing assembly for focusing the image of the ocular fundus as afunction of the image itself; g) a further beam splitter for reflectingthe image to said focusing assembly; h) a further video sensor forrecording the focused image; and i) the aerial focal plane of said lensbeing between said further beam splitter and said focusing assembly andsaid focusing assembly including a) a further lens located at its focaldistance from the aerial focal plane, b) an aperture located at the backfocal plane of said further lens, c) a yet further lens for providing afurther image, and d) said further video sensor being located at theback focal plane of said yet further lens for receiving the image of theocular fundus section under examination.
 2. The apparatus as set forthin claim 1 including a mounting for supporting a lens of the patient'sglasses intermediate said lens and said aperture.
 3. The apparatus asset forth in claim 1 including means for translating said apertureorthogonally to the optical path of the image between two positions toobtain sequentially two images forming, in combination, a stereo imageof the ocular fundus.
 4. Apparatus for imaging the ocular fundus, saidapparatus comprising in combination: a) a source (S) of illumination forilluminating at least a section of the ocular fundus (14) of a patient;b) a first lens (L1) for directing the illumination through a firstaperture (A1); c) a filter (F) for controlling the bandwidth of theillumination emanating from said first aperture (A1); d) a mirror(M1)for finely aligning the optical axis of the illumination, saidmirror including means (10) for repositioning said mirror in two axes;e) a lens (L2) for collecting the illumination reflected from saidmirror; f) a lens (L3) for receiving the collected illumination andforming an image of said aperture (A1) in the pupil of the patient toilluminate a section (12) of the ocular fundus (14) of interest; g) afirst beam splitter (BS2) for receiving the image of the section (12) ofthe ocular fundus (14) of interest and reflecting the image to a secondcollecting lens (L4); h) a lens (L5) for focusing the image receivedfrom said first beam splitter (BS1) upon a first video sensor (C1); i) asecond beam splitter (BS3) for reflecting a part of the collected lightto a lens (L6); j) a second aperture (A2) for passing the image fromsaid lens (L6) to a lens (L7); k) a second video sensor (C2) forreceiving the collimated image from said lens(L7); and l) means (20) forrelocating said apparatus in three axis to grossly align the opticalaxis of the ocular fundus (14) illumination with the pupil of thepatient.
 5. The apparatus as set forth in claim 4 including a source oflight (FIX) and a third beam splitter (BS1) disposed in the path of thecollimated illumination from said first collimating lens (L2) to permitthe patient to fixate upon said source of light resulting in exposure ofthe section (12) of the ocular fundus (14) of interest to theillumination.
 6. The apparatus as set forth in claim 5 including aplurality of said sources of light (FIX) at spaced apart locations tocorrespond with different sections of the fundus being exposed to theillumination.
 7. The apparatus as set forth in claim 4 including aplurality of photodetectors (PD) for providing a signal responsive toiris reflection.
 8. The apparatus as set forth in claim 4 including afurther source of light (16) for illuminating the eye of the patient. 9.The apparatus as set forth in claim 4 including a focusing assembly (FA)translatable as a unit for focusing the image upon said second videosensor (C2), said focusing assembly comprising said lens (L6), saidsecond aperture (A2), said lens (L7) and said second video sensor (C2)and means (18) for rectilinearly translating said focusing assembly. 10.The apparatus as set forth in claim 4 including a mounting (29) forsupporting a lens of the patient's pair of glasses adjacent said secondaperture (A2).
 11. The apparatus as set forth in claim 4 including means(24) for translating said second aperture (A2) orthogonally to the axisof the image passing therethrough between a first and a second positionto obtain a pair of stereo images having a stereo base equivalent to thedistance between said first and second positions.
 12. The apparatus asset forth in claim 4 wherein said source (S) of illumination providesillumination having a wavelength in the infrared region.
 13. Theapparatus as set forth in claim 4 wherein said source (S) ofillumination provides illumination having a wavelength in the nearvisible infrared region.
 14. The apparatus as set forth in claim 4wherein said source (S) of illumination provides illumination having awavelength in the visible light region.